1. Field of the Invention
The present invention pertains to downhole dipole seismic sources and more particularly to downhole seismic sources used to generate seismic shear waves.
2. Related Prior Art
It has been adequately demonstrated that dipole sources are capable of injecting detectable shear waves into formations surrounding liquid-filled boreholes. Foreign technology has produced a model tool, while two domestic corporations have reported successful experimental tools using dipole sources.
These three dipole source concepts mentioned above all use piezoelectric transducers, which generate relatively weak signals at frequencies of 4000 Hz and above. All three tools produce short bursts of energy and measure travel times between spaced receivers to determine formation shear wave velocities.
For example, U.S. Pat. No. 4,649,525 titled "Shear Wave Acoustic Logging System" (Angona et al.) describes a dipole source employing a piezoelectric bender type transducer as a point source of an acoustic shear wave. The bender type transducer described includes opposed unrestricted planar surfaces mounted within a liquid filled compartment within a logging tool. The unrestricted surfaces of the transducer are spaced longitudinally along the axis of the tool and exposed to the coupling liquid. One or more bender type transducers may be employed as acoustic receivers in these tools. The active surfaces of the receivers are oriented in substantially the same direction as the active surfaces of the transmitting transducer. U.S. Pat. No. 3,382,841, "Flexural Disc Transducer" (Bouyoucos), relates to a sound generator embodying the invention which makes use of a flexural disc radiating element which provides large acoustic power handling capability without the use of piston or thin spherical shaped wall members. This flexural disc radiating element also provides an acoustic transformer for transferring acoustic energy from a relatively small cross-sectional area to a relatively large cross-sectional area so that the impedances of an acoustic energy source and a load can be matched to each other.
Additional United States patents which illustrate the state of the art in dipole sources are as follows. These patents relate to hydraulic type acoustic wave generators.
U.S. Pat. No. 4,049,077, "Seismic Vibrator Control System", (Mifsud), relates to a control system for a hydraulic vibrator that permits the injection into the ground of seismic signals having pseudo-random code patterns or other broad band signals. Feedback for controlling vibrator operation is taken from the coupling plate. At low frequencies, the feedback is proportional to coupling plate position and at higher frequencies, the feedback is proportional to the coupling plate velocity.
U.S. Pat. No. 3,384,868, "Marine Vibrator Device", (Brown, et. al), relates to an apparatus for the generation of seismic energy waves within a water medium which consists of a rigid frame member and suspension attachments, the frame member movably securing a piston plate between upper and lower sealed bellows which are alternately pressurized by reciprocal oil flow to vibrate the piston plate relative to the frame member. The apparatus also includes an additional bellows which is sealed between the piston plate and frame member to receive varying air pressure to thereby maintain static pressure balance.
U.S. Pat. No. 4,147,228, "Methods and Apparatus for the Generation and Transmission of Seismic Signals", (Bouyoucos), relates to an apparatus for generating and transmitting seismic signals for the purpose of seismic exploration. The signals are force pulses shaped to have a spectrum constrained to the range of frequencies which are necessary for penetration to desired depths within the earth and for resolution of the geological reflection surfaces therein. These pulses are provided in a non-repetitive or aperiodic train, constructed to produce a transmitted energy spectrum whose mean energy extends smoothly at a substantially constant level over the spectrum frequency range, notwithstanding that the repetition frequency of the pulses may be swept over a frequency band much narrower than the spectrum range. The transmitted spectrum can exhibit an auto-correlation function having a major lobe which is predominant over any side lobes, corresponding to a desired level of resolution of the geological reflection surfaces.
U.S. Pat. No. 4,383,591, "Apparatus for Generating P Waves and S Waves", (Ogura), relates to an apparatus for generating P waves and S waves in a ground formation for a geological survey and the like. The apparatus has a percussive device which is driven to abruptly increase water pressure within a bore hole of the ground formation.
U.S. Pat. No. 4,483,411, "Tunable Marine Seismic Source", (Mifsud), relates to a marine seismic source which produces a varying FM signal in the low end of the acoustic spectrum. The seismic source uses stiff oscillating radiators to create a signal in the water. These radiators are attached to devices acting as springs with a variable spring rate. Variation of the spring rate as a function of the frequency permits the device to be tuned for maximum power output.
U.S. Pat. No. 4,578,784, "Tunable Marine Seismic Source", (Mifsud), relates to a device which is a marine seismic source which produces a varying FM signal approximately in the 10-100 Hertz region of the spectrum. The seismic source uses stiff oscillating radiators to create a signal in the water. These radiators are actuated by hydraulic cylinders which are in hydraulic communication with hydraulic or oleopneumatic variable devices acting as springs with a variable spring rate. Variation of the spring rate as a function of the frequency permits the device to be tuned for maximum power output.
U.S. Pat. No. 3,221,833, "Geophysical Borehole Apparatus", (Malmberg), relates to an apparatus for generating seismic waves. Geophysical prospecting is a technique employed to determine the subsurface structure of the earth, such as the geometry of sedimentary strata. This information is a valuable guide to the location of natural resource deposits, such as ore deposits, petroleum deposits, etc. The technique comprises the transmitting of vibratory energy into the earth and recording and interpreting the vibrations that return to the surface after reflection from, or refraction along, boundaries between geophysical formations.
While prior art has illustrated many devices for generating acoustic type seismic waves in a fluid environment, none have demonstrated the reliable generation of seismic shear waves in a frequency range close to surface vibrators.